In the communication system applied to the occupant protecting device for vehicles, a collision is generally detected by the acceleration sensors each provided in the main ECU and the satellite ECUs, and signals are exchanged between the two to process the result of collision determination or temporary data for the crash in the main ECU.
FIG. 1 is a schematic perspective plan view showing the positional replacement between the main ECU and satellite ECUs on a vehicle. 10 is a vehicle, 11 is the front portion, 12 is the rear portion, and 13a and 13b are the side portions. A main ECU 20 is installed in the central portion of the vehicle 10, and side satellite ECUs 30a and 30b are installed on the side portions 13a and 13b, respectively. Further, at both sides of the front portion 11, there are provided front satellite ECUs 40a and 40b, respectively. The main ECU 20 has a function of detecting a collision occurred at the front side of the vehicle to unfold air bags in front of occupants, particularly the driver and the passenger-side occupant, and a function of receiving collision information from the other satellite ECUs 30a, 30b, 40a, and 40b to unfold the air bags on the side or in front of occupants. The side satellite ECUs 30a and 30b are to detect a side-impact collision by an acceleration sensor, and has a function of determining the accuracy of the collision by its own microcomputer and sending it to the main ECU 20 through communication lines 31a and 31b. Further, the front satellite ECUs 40a and 40b are provided, because an offset collision may not be determined only by the main ECU 20, and they have a function of processing the detection status of the acceleration sensor provided in them by their own microcomputers, and sending the processed data to the main ECU 20 through communication lines 41a and 41b. 
Data sent from the satellite ECUs 30a, 30b, 40a, and 40b to the main ECU 20 represent the determination result of collision and the fault diagnosis status of the acceleration sensor under normal conditions. Thus, in the conventional communication system, a trigger signal functioning as a data request command is sent from the main ECU 20 to the satellite ECUs 30a, 30b, 40a, and 40b, and the respective satellite ECUs 30a, 30b, 40a, and 40b transmit a collision determination result or a fault diagnosis result to the main ECU according to the trigger. The main ECU 20 processes and determines the transmitted result, displays a warning or unfolds an air bag.
FIG. 2 is a block diagram showing the schematic circuit configuration for implementing the conventional communication system. In this figure, the required minimum main ECU 20 and satellite ECU 30 in a pair are shown for simplicity.
In the figure, 1 is the battery of the vehicle, by which a d.c. current is supplied to the booster circuit 21 of the main ECU 20 via an ignition switch 2. 3 and 4 are squib resistances for igniting the gun powder for unfolding the air bags (not shown) for protecting the front portion and the side portion of an occupant. If a collision occurs at the front side of the vehicle, an ignition current is supplied to the squib resistance 3 from the booster circuit 21 through a mechanical acceleration switch 27, which closes when it senses an impact in the longitudinal direction of the vehicle, and a switching transistor 28 controlled by a microcomputer 23. To the other squib resistance 4, an ignition current is also supplied from the booster circuit 21 through a switching transistor 29 controlled by the microcomputer 23 upon the occurrence of a side-impact collision to the vehicle. Further, from the booster circuit 21, a d.c. voltage is supplied to fixed-voltage circuits 22 and 32 functioning as the d.c. power supplies for the microcomputers 23 and 33 and other circuits.
Now, the operation is described.
The microcomputer 23 always determines faults of a longitudinal acceleration sensor 24 in the normal condition where there is no collision, and if a fault occurs, it outputs a signal for controlling an alarm (not shown) such as a lamp. If a collision at the front side of the vehicle occurs, the microcomputer 23 determines the detection signal from the longitudinal acceleration sensor 24, which represents a collision state, and outputs a control signal to bring the switching transistor 28 into conduction. At this point, if the mechanical acceleration switch 27 is closed, an ignition current is supplied to the squib resistance 3 to unfold the air bag in front of the occupant.
Further, the microcomputer 23 exchanges signals with a communication circuit 25. Based on a clock pulse, a trigger signal (a) of FIG. 3 of a fixed period is sent from the communication circuit 25 to the side satellite ECU 30 side through a communication interface 26. As described later, a signal (b) or (c) of FIG. 3 sent from the satellite ECU 30 in response to the trigger signal is provided to the microcomputer 23 through the communication interface 26 and the communication circuit 25.
In the satellite ECU 30, the microcomputer 33 performs the normal fault diagnosis of a lateral acceleration sensor 34 and an acceleration switch 37, and determines the detection outputs of the lateral acceleration sensor 34 and the acceleration switch 37, if a collision at its lateral side of the vehicle occurs. In the fault diagnosis, the signal (b) of FIG. 3 is sent to the main ECU 20 side via a communication circuit 35 and a communication interface 36, and if the microcomputer 23 determines that an abnormal state has occurred, the alarm is operated to alert the occupant to it. Further, in the event of a side-impact collision, determination is made in the microcomputer 33 upon receipt of the detection signals of the lateral acceleration sensor 34 and the acceleration switch 37, and the signal (c) of FIG. 3 indicating a collision is sent to the microcomputer 23 through the communication system. Upon receipt of this signal, the microcomputer 23 determines whether the collision has actually occurred, and outputs a control signal to turn on the switching transistor 29 if it determines that the collision is dangerous. Whereupon, an ignition current flows through the squib resistance 4 via the switching transistor 29 to unfold the air bag for side protection.
The conventional communication system is described according to FIG. 3. A trigger signal (a) of repetitive pulses with a fixed period TO is always sent from the main ECU 20 to the satellite ECU 30 side. On the other hand, a diagnosis data provided by checking the fault status of the lateral acceleration sensor 34 and the acceleration switch 37 are outputted from the microcomputer 33, and upon the reception of a predetermined number of pulses of the trigger signal (a), a response signal (b) including the diagnosis data is sent from the communication circuit 35 to the main ECU 20 side. The response signal (b) is provided to the microcomputer 23, which determines the diagnosis data, and issues a control output to drive the alarm if there is anything wrong. Further, if a lateral collision occurs with a vehicle, the response signal is as shown by (c).
Since the conventional communication system is constructed above, the main ECU requires a transmission process for always sending the trigger signal, and a data reception process for the response signal, so the processing is complicated. Furthermore, there is a problem that the satellite ECU requires a circuit for receiving the trigger, causing an obstacle to the downsizing of the device.
This invention was accomplished to solve the above described problem, and its object is to obtain a communication system in which the communication between the main ECU and the satellite ECU is carried out by the start-stop synchronization communication provided in the microcomputer.